NHK reported the full study of Prof. Watanabe, which Yomiuri newspaper missed.
The researching group of Prof. Watanabe assumes the radioactive particles circulate the world taking 40 days.
They measured 0.0048 Bq/m3 of cesium in May 2011, but it was 0.0007 Bq/m3 this March.
The radioactive particles travel around the world and fall off to the ground to decrease its concentration.
Prof. Watanabe comments, they can trace how radioactive particles spread over the world and how to affect each country on its path.